Not Applicable
1. Field of the Invention
The invention relates generally to the fields of x-ray computed tomography (CT) studies and single photon emission tomography (SPECT) nuclear medicine studies, and in particular, to a computed tomography system suitable for simultaneous transmission x-ray computed tomography (CT) studies and single photon emission tomography (SPECT) nuclear medicine studies, with substantially increased spatial resolution.
2. Description of Related Art
Traditional computed tomography detectors employ silicon diodes or photo multiplier tubes coupled to scintillators. These detectors operate in current mode where the product of the mean x-ray event rate and the average x-ray energy is the measured parameter. These detectors are well suited for high count rate studies where high x-ray fluxes are employed, but they do not provide information about the energy of the individual x-ray energies. As a consequence, no energy dependent processing (such as multi-spectral image analysis or scatter correction) may be performed. The system disclosed here operates in xe2x80x9cpulse modexe2x80x9d[2] where each x-ray is individually counted and its energy is recorded. The inclusion of energy (or spectral) data in the image provides improved soft tissue differentiation, reduces the effects of beam hardening and permits some degree of correction for scattered x-rays in the image.
Photon counting systems have been developed elsewhere using pixelated high purity germanium (HPGe) detectors. The, HPGe systems have many of the advantages described here, but require detector cooling, typically to 77xc2x0 K. Pixelated detectors also have the disadvantage of requiring an electronic channel for each pixel. The CZT detectors employed here operate at room temperature and require less volume than HPGe due to their higher atomic number. Furthermore, the strip detector configuration requires only 2N electronic channels for N2 pixels.
A novel computed tomography system suitable for simultaneous transmission x-ray computed tomography (CT) studies and single photon emission tomography (SPECT) nuclear medicine studies overcomes the problems of the prior art. The system employs cadmium zinc telluride (CZT) two-sided strip detectors (2SSD) to detect both x-rays and gamma rays (xcex3-rays). The x-ray CT measurements provide very high resolution images ( less than 50 xcexcm FWHM) of the laboratory animal""s physical structure while the SPECT measurements provide lower resolution (xcx9c5 mm FWHM) functional images of the laboratory animal""s metabolic activities. Both the CT and SPECT images are acquired in xe2x80x9cpulse modexe2x80x9d where each x-ray or xcex3-ray is individually detected and its position and energy are individually recorded. Because the same mechanical assembly and detector are used for both CT and SPECT studies, the two types of measurement may be performed simultaneously and displayed in a single image. Previous technologies for combining images from different imaging modalities required two separate imaging systems and complex algorithms for post-acquisition registration of the images. Furthermore, unlike traditional CT data acquisitions which do not record the energies of the individual x-rays, this system can perform multi spectral analysis of the x-ray data set providing an opportunity for beam hardening correction and separation of soft tissue image data from skeletal tissue data. Finally, the 2SSD detector configuration requires only 2N electronic channels per N2 pixels compared with standard pixelated detectors requiring one electronic channel for each pixel.
Researchers at Brookhaven National Laboratory have used monoenergetic x-ray beams from synchrotron x-ray sources. They have demonstrated that the use of monoenergetic sources improves image contrast, particularly for soft tissue, and eliminates artifacts due to broad x-ray spectra. Synchrotron sources are large and very expensive, however, and poorly suited for clinical use.
This is believed to be the first proposed use of CZT 2 sided strip detectors for computed tomography. This system will increase the spatial resolution by an order of magnitude over the only other reported dual x-ray CT/SPECT system. This system will extend the state of the art in multi energy computed tomography.
A method for simultaneous transmission x-ray computed tomography (CT) and single photon emission tomography (SPECT), in accordance with the inventive arrangements, comprises the steps of: injecting a subject with a tracer compound tagged with a xcex3-ray emitting nuclide; directing an x-ray source along an axis toward the subject; rotating the x-ray source around the subject; operating the x-ray source during the rotating step; rotating a cadmium zinc telluride (CZT) two-sided detector on an opposite side of the subject from the source; simultaneously detecting, with respect to position and energy, each pulsed x-ray and each emitted xcex3-ray captured by the CZT detector during the rotating; recording data indicative of each the position and each the energy of each the captured x-ray and xcex3-ray; and, creating respective CT and SPECT images from the recorded data. All of the steps can be implemented at ambient temperatures.
The method can further comprise the step of limiting the captured xcex3-rays to those of the xcex3-rays emitted along a predetermined set of projection angles, for example by collimating the xcex3-rays at a position in front of the detector. The projection angles can be normal to the detector.
The method can comprise the step of detecting and recording a pixel position, an angle of rotation and an energy level for each captured x-ray and xcex3-ray.
The scanner field of view can be set by adjusting the width of the detector in the plane of rotation.
The method can comprise the step of capturing the x-rays and xcex3-rays with an array of orthogonal stripes of the CZT. Biasing the stripes with an electric field, some of the stripes become anodes and the others of the stripes become cathodes, whereby electrons drift toward the anode stripes and holes drift toward the cathode stripes. The position of each captured x-ray and xcex3-ray can be established by determining which ones of the anode and cathode stripes carry a current pulse resulting from the captured x-ray or xcex3-ray.
The method can further comprise the steps of: determining ratios of electrical charge collected in adjacent ones of the stripes; and, determining each the position based on the ratios.
The method can alternatively comprise the step of capturing the x-rays and xcex3-rays with an array of pixelated CZT detectors.